Signal shaping amplifier



March 27, 1951 w. A. ANDERSON SIGNAL SHAPING AMPLIFIER 4 Sheets-Sheet 1 Filed Jan. 31, 1946 nu Dubbin- INVENTOR WARREN A. ANDERSON & v

BY ATTORNEY Man}! 7, 951 w. A. ANDERSON 2,546,813

SIGNAL SHAPING AMPLIFIER Filed Jan. 31, 1946 4 Sheets-Sheet 2 1 v Him/may I Imnentor WARREN A. ANDERSON (lttorneg March 27, 1951 w, ANDERSON 2,546,813

SIGNAL SHAPING AMPLIFIER Filed Jan. 31, 1946 4 Sheets-Sheet 3 @7' Zinnentoq I WARREN A. ANDERSON (Ittomeg March 27, 1951 w. A. ANDERSON I SIGNAL SHAPING AMPLIFIER Filed Jan. 31, 1946 4 Sheets-Sheet 4 Zinnentor i 57 VSZARREN A.ANDEBSDN A IA/: 7 My:

i (Ittovneg bias-as a function of the signal amplitude.

tion -to. follow.

output of the threshold amplifier. 'stant weight and amplitude signals are delivered to the outgoing line.

Patented Mar. 27, 1951 signor to Radio Corporation of America, a cor- ,poration of Delaware Application January fl,1946; Serial No.s44-,-520' *8 Claims. (01, vs -m lff --This inventionirelates to signal shaping .am- .2

"plifirs and has 'for its principal object to proi'vide an amplifier system which is simple in con"- -struction and which is effective in. translating mutilated telegraph signals into readable on-off signals of rectangular shape, constantamplitude and constant weight. By weight I refer to the ratio between the markin time and. the spacing time of a signal having dot unit length. In long distance radio telegraphy the signals picked'up by the radio receiver are very often 1 severely distorted Atmospheric conditions cause the signals to' be received by diverse paths and quite often to be elongated. .Other causes contribute to the mutilation. In order to over- ,come these effects various wave shaping systems have been devisedin the past." The majority of these systems employ cut-off tubes to obtain a threshold value for the amplitude of the signal. .Limitingamplifirsj have also been provided for maintaining constant the amplitude of the .out-

in a receiving system a signal shapingamplifier a characteristic of which is to vary the threshold 1 Another object of my invention is to provide a signal shaping amplifier in which the mark- Other objects and advantages ofm-y invention I the past, it is an objectof my invention to provide will be made apparent in the detailed descrip- This description is accompanied by a. drawing, wherein Fig. 1 represents. a

preferred circuitarrangement for carryingout vtheinvention; Figs. 2,3 and4 each illustrate a Briefly stated, the invention employs a threshold amplifier stage the bias of which is developed series of curvesgive'n in'explanationof the operation of the invention.

by a long-time'constant peak measuring voltindicated as -|-C and C respectively,-the

former 20' is connected... The transformer secondary is connected to aload consisting of two potentiometers 22, theiinterconnected; ends ofiwhich are grounded. A center tap on the secondary winding of transformer 20". is also grounded.

Two amplifier tubes 2 are connected" in push r,

:pull fashion and are provided with input .circuits which include taps on the-potentiometers 22. The output circuits of'these tubes include anode circuit resistors l3 throughwhich anode potential is supplied from a direct current source indicated by the -i-B terminal. 'The'L-B ten minal of-this sourceisgrounded. 1 'F .:"-1j=:

Cathode resistors-4 are connected between'ithe respective cathodes- 0f the tubes'JZ and .grormd. These cathode resistors are common: to. the .1111- put and output circuits. N ----Discharge tubes 5-: arealso connectedfinpushzpull l fashion and possess I input 1 and :oufiputlicir cuits 'whichare characterized 'as follows'z'. The cathodes are connected to ground through "cathode resistors 2-5. The anodes are supplied with directcurrent from the I-B source..whicl1 -is connected to a mid-tap on the primary winding of a transformer 2|. The terminals of thisp'ri mary' are connected respectively to one and the other of the anodes inthe tubes 5. The control grids in these tubes are individually coupled to the respectiveanodes in tubes 2 by capacitors 23.

The grids in tubes 5 are normally maintained at a bias" which" is negative witlirespect totheir associated cathodes. Thenegative Value of" this bias is equal to the voltage drop across two re"- sistors Sand 1 which connect the junction between the two resistors 24 to ground. Resistor I in serieswith resistors-'8- and 9- constitutea voltage divider or bleeder resistor and are connected across the terminalsof a biasing source terminal being grounded. q

Resistor 6 serves as a load to amplifier l4 whose cathode is connected through a'cath'd resistor l5 to the junction 'between resistor s 8 and 9. The input circuit of tube [4 includesa resistor [3 in shunt with a capacitor IZQcqm} ponents l 2 and l3 beingconnected between the control grid and the C source terminal. The

grid of tube- I l" is also directly connected to are limiter amplifier placesa maximum limit on the Thus con,-

- Referring to the drawing, I show input terdeq a i d ree ifl r t b .U. The- 1 tion of this tube will presently he explains An amplifier discharge tube I'll-has its cathfode connected to ground through resistors 3;] and 32. Its input circuit includes resistor 33 which is connected betweenthe junctiongof' re- Qsistors '3! and 32 and the control grid. Tube receives its control (that is, receives its excitation) from the anode inone of the tubes 2, this anode being coupled to the grid in tube Ill by a capacitor 2Q. The anode in tube Iii is supplied with +B potential. ihe junction between resistors 3| and 32 is coupled to the anode in tube H by a circuit consisting of an inductive choke 34 and a capacitor 30.

The output circuit from the push-pull tubes 5 is coupled across transformer 2| to the input circuit of tubes l6 which are connected in push-pull fashion. The cathodes of tubes l6 are both grounded, as is the center-tap on-the secondary winding of transformer 21! The terminals of this secondary are connected individually to the grids in tubes I6, connections made through resistors 21. includes the primary winding of a transformer 28. This primary winding is connected at its midtap to the +3 source. The secondary winding Tot transformer 28 is connected to output terminals and delivers a keyed tone signal, one character.- isticof which is that it possesses constant ampli 151166 for marking pulses.

The operation of my improved signal shaping amplifier will now be explained. The principal feature or the invention is the means for varying the bias on the grids of the push-pull tubes 5. means includes the circuit arrangement of tubes. l0, I: I and i4 and their associated circuit components.

- As before stated, tube Ni derives its control (that is, receives its excitation) from the anode niche of the tubes 2.. The variations of voltage drop in the cathode: resistors 31 and 32 are reflected in a variable amplitudeof current rectifled by the diode tube H. The efiects of this rectification are, however, smoothed out by means of the time constant circuit consisting. of. resistor l 3: in shunt with the capacitor i2..

Tube M- derives its control fromv the cathode voltage in tube H. Tube it acts as a shunt of variable resistance with. respect to the resistor a in the biasing circuit. for the grids in the tubes 5.

The output from amplifier tube H1 is adjusted to some. predetermined percentage of its input. In practice, this adjustment has been found to be an optimum when it isless than 100' per cent. The. variable charge on condenser 12 is arranged 'to satisfactorily control the conductive state in tube 14 by virtue of arelatively high resistance value which is chosen for resistor 13. The electrode of capacitor l2 which is connected to the cathode of tube ll tends to be driven positive andv thus renders tube l4 more or less conductive.

Other components of my improved circuit are so. chosen that when the input signal is of zero value there is no voltage drop'across resistor i3 and. thiscondition renders tube [4 non-conduc- .SignaL. g In the presence of an. incoming marking, sig- 11-31,;- the cut-E bias applied to tubes is varied as a function of the input signal amplitude. The control voltage applied through capacitors 23 however, 'sufiicient to render tubes 5 conductive order to translate the input marking, signal into useful energy for delivering signals. in the 4 output circuit at constant amplitude. Thus, the signal amplitude itself varies the thresholding bias on the input circuits of tubes 5. Theieffect of operation of the circuit is to maintain in the output signal from tubes 5 a constant ratio between mark and space signal components of unit length despite variations in input level.

The output of the threshold stage 5, Sis transformer coupled to the push-pull limiting amplifiers l8 which complete the signal shaping process and deliver constant amplitude tone signals to the outgoing line or utilization device.

The output circuit from tubes l6 The operation of applicants invention will now be explained with reference to the curves of Figs.

-2, 3 and 4. Figure 2 shows three different signals A, B and. C with various weights. The letter M represents the length of the marking element on the same horizontal line therewith, while the letter S represents the spacing element on the same horizontal line. with this last letter. Signal A is a transmitted signal consisting'solely -of reversals; each baud which is of unit length, d. Tlfis unit length, d, is a function of keying speed and method. of transmission. For single channel, start-stop operation at 62 words-per-minute, d is about 22 milliseconds; while at 100 words-per,- minute,,d is about 13.5 milliseconds. The rate of reversals is the keying frequency f and t is equal to 1 frequency or the time for one cycle; Since is the time for one-half cycle, then t=-2d.

Signal B represents a received signal where the marking element has been. elongated. due to multipath, etc. Since the; marking element in signal 13 is 1.511 or .75t and. the spacing is: .512 or .2515, thissignal is referred to as having a weight of /25.. I

Signal 0 represents a. received signal where the marking has been shortened due to multipath, etc. Since the marking is .25 5 and the spacing is .7515, this signal is referred to as having a weight of 25/75.

Referring to Figure 1 of the drawing. it isv evident that if the connection from the anode of tube 2 to the condenser 29 is broken and if the junction of the two resistors numbered 24 is grounded, there will result a straightforward amplifier. Doing the above eliminates the efiect of tubes l0, II, M and associated components.

In prior systems, in order to achieve thresholding the junction of the resistors 24 would be connected to a battery of minus polarity instead of ground. The use of a fixed threshold voltage on the grids of 5 caused the weight of the output signal to vary with input signal'amplitud'e. This is shown in Figure The letter K shows the point. at which tubes 5 start to conduct and the letter Y shows the point at which tubes. '5 just stop conducting.

. In Fig. 3, the signal of line' A shows a received signal. The tone measures 2'0 volts peak-to peak or Ill-volts each side of. zero. This is for marking.

Line C of Fig. 3 shows a signal with the same weight as line A, but the amplitude has increased to 40 volts peak-to-peak. With the fixed threshold .voltage of 5 volts, the output signal now has a weight of 60/40, as shown in line D" of Fig. 3.

Line E of Fig. 3 shows a received signal whose l2$$43818 5 i6 dviser-i1: "is thesameras thesignal df'line ""Eig. 11: wnl bewmderstouwbyxhose skilledinthe i'amt :srtur wmse amplitude i s 'BO VQIts peak-to ak. that thutput terminals of :n-y impi v d sisn 37931211 'tl're fixed threshold Vo1tage of- 5 *iol yt'he sha'pin m-plifiermay' -he rconnected 1th rany'aleoutput signal now has a weight of 67/33, as shown 'sired zutilizatiori 'device. 1 Ordinarily; "the r-output tenderer-mg. 3. 5 would be rectified andiappliedito-alecliingnircuit ""If we now proceed to liminate the 'hattery and eith'er directly-or through 'a telegraphdistributor.

re' fzonnedt condenser 29- to the anode oftu'be v2 various imedification's of iny'improved icircuit iand then'ction o'f'the resis'tors i l to th'e' anode arrangement-the preferred form of which iis o'fi -tiibe M nd' res'i'storfig'wvehave the =oircui-t :of herein showmamay, of conrseybemade without -th"pres'en venti'on. Thisoircuitcan:he con- '10 departing from the ispir'it andscope-of the-tin- -Sid'ered as a ibatter'y'with axvetrYing-Voltage. The 'vention.

i 'tery vol ta'ge' will be acertein predetermined I 1 151231111:

ortionnf the inDutEsignaL- ThistMeshold-wolt- 1. 11159; telegraph iteoeiving system, aetelegraph sage" =wil1 var-y with input signal voltage; 'ias Gsignal shaping -ei-rcuit arrangement of the i l 1=5 whichsubstantially-restores the original rectanf- ZTI'IBI-IHDIE i'si'gnal ,fs' 'eo'upled' from I the'fan'odenf gul'ar pattern of con-off keyed 'tone signals, and

:tiib'e:2:thrdugh bondenser 29"tolthe g-ridfoftube main'tainsmonstant :the weight of the marking ljlL-iELhe output oftubeul il i's eoupled through eonelement's theneofjtsaid arrangement comprising denser wito thwectifierstage. The mitput of g plurality of at'least three cascaded amplifier 4511M I UiS-"aIWaYS SOmQ f'lactio 0f its input T1; stages-means iicr sup-rilying the input ofzsaid 'first ean never-' h61'00% 0'r m0re.' The d 'd i cstagewith telegraph signals transmitted from a tion can he achievedcbyiproper selection of resispoint an whi h m be distorted by the rbbrsfil Iand 32. The'irectifier stage I:|.cav.ses'con- -medi=um verwhich they pass, means for variably denser til-"t0 be positively c ed'and sincei'recontrolling the value of the direct current-bias fi'si'stor le' has'relatively highresistanceycondenser 'p0tentia1:"app1i'ed to t ljnput e t -i 4 1 p p e'c a l e timain "secondsa'id amplifier sta e a's a function' of .the' *wmparisonlto-vthe time of keying cycle: The output potentials'derived' from the firstcstage, woltage 'across condenser L2 determines'the-=cur uafndr'meansifoi" producing :a limiti g a ti -si rent flow through" tube" I l-rand resistor '5. The t t mf idrOD aorosszresistor Bde'termi-nes the bias on- 1 signalwshapmg amplifier comprising; has a gain r "translating circuit arrangement zhaving a-- rfi=rst "on tubes :5 is a function of the voltage across :congpush-apull amplifi r gs'jbagefgonrlthe input c sf l 131w Voltage "MOSS wndenser is which .keyeid telegraph" zsi-gnal's'care impressed; a @a i qnvoltage of the ti signals *seond liush pull' -ampl-ifier" stage eou md to-saia m u'p d Omfllfl th ffiu fi 2 :first stage andsubjeettocontrol'by-aoutputgrrergy s since theam-plitude of the tone from said-zfirst- 'sta'ge, :a iimrrin'gempiifier esrage si s up d through condense-r7397 is 21 F edisposedzin acasade relation'tosaidiseOnd-nm- -:-i:letermined portion of the amplitude-Jot the inpfifier m {means coupled mg-engmmp t gput signals, it is evident that the threshold-bias w stage f r varying ithccutsofilbials n n' the grids of- 1S p wq i '40 'potentialiappliedto the-input "electrodes Unsaid s when there is mput secondamplifier'stageas a 'funetionof theam'pli- *IIaL IBSIStOIS 1, 8-, 9 and -'|5- areof such value tude f signal/output f o the fi sfust ge ww venoflgh'voltage across resistor is to 3 11st "said "means includingelectronic devices for amukeepstubesfs blasedoff' v inlifica-tion, rectification and forproducing a "Figure thls *3 'variable' i'imnedance 'in' series with a source of .'-a,ge-he'1ps -to -ma-iritain an output of constant .,bi,asing.potehtial-for,saidzseccndgstaga weight When -thein-nut vsignal weight remains U '3; Shaping. circuit. for-keyed tone ;sina1s constantJout its ainplltude v'ar comprising at least three cascaded push-"pull ThB SIQHfiJ-S 0f llnes wA, and of'Flgule are amplj fierr rsta ge -o'f, the'first controlgi "i n Signals l A, C and E 0 second; I and the seco'nd controls "the third; a

Figure 'T fi $1 8 1111911tv Signal with Similar direct current source --having a'voltage' divider mig are used In b -e connected-a'cross itsterminals, a circuit of varif 4 Line of Figure 4 shows a receivedsignal of 20 ,1 1 .1 mlmdance interconnecting two p t i o peak-tomeak of hr -said voltage-divider-and including the space rdisfi -itiisqassumed that the assoc ated components i-y-afihcharge pathof anglgctrgn-tub rm 1; r t I :o f tube zm are such as nto-give 5 of the input biasing icircuitfor-thz eondseiid'amplifi'rfstage, slg' l a s fl Voltage, than file-threshold said last circuit-inoluding' said ircuit of vari'ab'le w ta at the tn I'51s v s impedance,--means=c0uri1ed to SaidfirSt stage for coincides with the fixed threshold voltage "amplifying dfrgctifyjng,jai"COinponent:df-= jgna1 are2*andftheweight ofiili u p ig l i fi wuenerg derived from the first-said ush-pull in both cases, as shown in line i5. --stage;nonnectionsiorifeeding the product-ofsaid Returning toFigure 4; line 'Q the input signal y et ilt dmp t Hci1zcuit;ofw;said itb' -aaniplitude has increased tofiflrolts, the threshold lvth 'by "to -control--thesame,- where'by th level v ta isnow th hto'fh u pu s sor the iout -toif bias in .said .secend stage is varied na11s still 50/50 as shown in line D. e as a function-of .theincoming signal --.a-Irip1itude, Line E ofFigure 4'shows an input signal whose and means" in cireuit with said third stage-*for amplitude is now 30 volts, th'e threshold vol tage producing a limiting action in the mantra "is diow y-jlfi volts. l h'e' Weight of the output is'igp iisl'i pull.z'aiinp'lifur J'stage. 1 ilt l a h wnin i f f Fie-e- 43m 5 "devic'for translating distortedte le- "b w ii n 'Yialone theti'mea'z'iis orizontal) "having a substantially;rebtan u1ar envelo eand iii inrea'ses "with input signal "amplitude While from having -ac0rr'e'cted wei'ghtrelation "to the inter- Figur Li-t is seen thatth'edistance between X ve'ni-ng' *s'pace elements, a pnsh -pull "ahnzilifier and -remains constantfor ai1 =value"s0i input system having at least three concatenatedstages, fiig nal amplitude. v

. rem-Figure 3 it-can-be seen thjat the'a 'stal' ice'fiqo graphsignal's'into'tone' rnodulated in'arkelements 7 :catho'de biasing circuit for the second of said stages, said network includin a direct current source and a controllable discharge tube which possesses its own input and output circuits arranged for activation by said source, and means including an amplifier coupled to the output of said first stage and subject to control from the first stage in said system and including a rectifier having a time constant circuit coupled between said amplifier and the input of said controllable tube in said variable impedance network, the last said means being effective to regulate the bias potential which is applied to the input circuit of said second stage, said time constant circuit including a capacitor and a resistor in the input circuit of said controllable discharge tuba-said time constant circuit being efiective to produce relatively slow changes of biasing potential'applied to the input circuit of said second stage, and means for operating said third stage as a limiter stage, thereby to maintain said signal weight relation substantially constant.

5. A signal shaping amplifier system for telegraph signals, comprising first and second resistance-capacity coupled push-pull stages each 1 having a pair of Vacuum tubes, said tubes each comprising a grid, a cathode and an anode, means for supplying telegraph signals of varying amplitude in push-pull relation to the grids of said first stage, circuit elements for biasing said first stage to act as an amplifier for accepting said signals of varying amplitude, circuit elements for biasing said second stage to the anode current cut-ofi condition in the absence of signals applied 'to said first stage, a single amplifier tube having a grid and a'cathode, a connection from the grid of said last amplifier tube to the anode of one of said tubes in said first push-pull stage for receiving excitation therefrom, a rectifier coupled to the cathode of said single amplifier tube,-

vond stage, whereby the bias on said second stage is the sum of the cut-off voltage and a fixed percentage of the applied signal, and means for limiting the output of said second stage.

6. A signal shaping amplifier system for telegraph signals, comprising first and second resistance-capacity coupled push-pull stages each having a pair of vacuum tubes, said tubes each comprising a grid, a cathode and an anode, means for supplying telegraph signals of varying amplitude in push-pull relation to the grids of said first stage, circuit elements for biasing said first stage to act as an amplifier for accepting said signals of varying amplitude, circuit elements for biasing said second stage to the anode current cut-01f condition in the absence of signals applied to said first stage, an amplifier device having a grid and a cathode, a connection from the grid of said device to the anode of one of said tubes in said first push-pull stage, a half-Wave rectifier coupled to the cathode of said amplifier device and having a time constant circuit of resistance and capacitance in its output, a direct current amplifier stage coupled to and controlled by the output of said rectifier, connections coupling said direct current amplifier to the biasing circuit 7 element fo S id e d sh-p lstase. w e eby the biason said-second stage is varied asa function of the amplitude of the signals, and means for limiting the output from said second stage. 1 i:

'7. A signal shaping amplifier system for telegraph signals, comprisin first and second resistance-capacity coupled push-pull stages each having a pair of vacuum tubes, said tubes each comprising a grid, a cathode and an anode, means for supplying telegraph signals of varying amplitude in push-pull relation to the grids of said first stage, circuit elements for biasing saidfirst stage to act as an amplifier for accepting said signals of varying amplitude, a bleeder-resistance network connected from a point intermediate its end to the grids of the tubes of said second stage for biasing said second stage to c'ut-ofi in the absence of signa1s applied to said first stage, an amplifier device having a grid and a cathode and providing an output which is less than its input, a connection between said last grid and a tube in said first stage for supplying excitation potentials to said device, a resistor network in the cathode circuit of said amplifier device, a halfwave rectifier having an anode coupled to a point intermediate the ends of said last resistor network and having a cathode coupled to a-time constant'circuit, a D. C. amplifier having a grid coupled to the cathode of said rectifier and having cathode and anode electrodes coupled across a portion of said bleeder resistance network, said D. C. amplifier being biased to cut-off by the bleeder resistance network in the absence of signals, whereby the bias applied to the grids of said second stage in the presence of signals "is the cut-off voltage plus a fixed percentage of the applied signal. a

8. Ina telegraph receiving system, a telegraph signal shaping circuit arrangement of the type which substantially restores the original rectangular pattern of on-off keyed tone signals,'and maintains constant the weight of the marking elements thereof, said arrangement comprising a plurality of at least three cascaded push-pull amplifier stages, means for supplying the input of said first stage with telegraph signals which may be distorted by the medium over which they pass after being transmitted from a remote point, means for variably controllin the value of the direct current bias potential applied to the input electrodes of the second said amplifier stage as a function of the output potential derived from the first stage, said last means including a grid biasing circuit, the impedance of which is varied by inclusion therein of an electron discharge device through the space path of which at least a portion of the current for said bias potential is fed, and means for producing a limitingaction in'the third stage. .4 WARREN A. ANDERSON? REFERENCES CITED The following references are of record in the file of this patent: 

